1. Field of the Invention
The present invention relates generally to x-ray tubes and power supplies for x-ray tubes.
2. Related Art
A desirable characteristic of x-ray sources, especially portable x-ray sources, is reduced power consumption, thus allowing for longer battery life. Another desirable characteristic of x-ray sources is power supply electronic stability.
Power Loss Due to Filament Heat Loss
One component of x-ray sources that requires power input is an x-ray tube filament, located at an x-ray tube cathode. Alternating current through the filament can heat the filament to very high temperatures, such as around 1000-3000° C. The high temperature of the filament, combined with a large voltage differential between the x-ray tube cathode and anode can result in electrons propelled from the filament to the anode.
Some of the heat at the filament can be lost to surrounding components through conduction and radiation heat transfer. Electric power input to the filament is required to compensate for this heat loss and keep the filament at the required high temperature. This electric power input to compensate for heat loss results in wasted power and, for x-ray sources that use batteries, decreased battery life.
The wasted heat can be transferred to electronic components in the power supply, resulting in temperature fluctuations in these electronic components. These temperature fluctuations can cause instability in the power supply because of the temperature dependency of many electronic components.
Power Loss Due to Linear Regulator
Another component of x-ray sources that can cause power loss in x-ray sources is a linear regulator in an alternating current source for an x-ray tube filament. FIG. 7 will be used in the following discussion regarding use of a linear regulator 72 in an alternating current source 70 for an x-ray tube filament.
Voltage source 401 can provide direct current (DC) to a direct current to alternating current (DC to AC) converter 403. Voltage source 401 can be a constant voltage power supply. X-ray tube 405 is shown comprising a filament 406, cathode 407, evacuated cylinder 408, and anode 409. The DC to AC converter 403 can provide alternating current to x-ray tube filament 406. A transformer 404 may separate the DC to AC converter 403, at low DC bias voltage, from the filament 406, at high DC bias voltage, thus an AC signal can be passed from a low DC bias to a high DC bias. Due to heat caused by alternating current through the filament 406, and due to a large DC voltage differential between the filament 406 and the anode 409, an electron beam 410 may be generated from the filament 406 to the anode 409. Electrons from this electron beam 410 impinge upon the anode, thus producing x-rays 417.
There is often a need to change the flux of x-rays 417 exiting the x-ray tube 405. Adjusting alternating current flow through the filament 406 can change the electron beam 410 flux and thus the x-ray 417 flux. A linear regulator 72 can be used to adjust alternating current flow through the filament 406.
Electron beam 410 flux and thus x-ray 417 flux can be approximated by an amount of electrical current flowing from a high voltage multiplier 411 through feedback module 414 to a filament circuit 412. The feedback module 414 can determine the current flow, such as by measuring voltage drop across a resistor. The feedback module 414 can receive input 416, such as from an operator of the x-ray source, of a desired x-ray 417 flux. The feedback module 414 can then send a signal 415 to the linear regulator 72 to change the amount of current to the DC to AC converter 403 based on the input 416 and the x-ray 417 flux.
For example, input 416 can be reduced for a desired reduction in x-ray 417 flux. Feedback module 414 can detect that x-ray 417 flux is too high due to too large of a current through the feedback module for the new, lower input 416. A signal 415 can be sent to the linear regulator 72 to increase voltage drop across the linear regulator 72, thus allowing a lower DC voltage to reach the DC to AC converter 403. The DC to AC converter 403 can then provide less alternating current to the filament 406 resulting in lower filament 406 temperature, lower electron beam 410 flux and lower x-ray 417 flux.
The larger voltage drop across the linear regulator 72 at low x-ray 417 flux levels can result in wasted power because the power input from the voltage source 401 can be the same at low x-ray 417 flux as at high x-ray 417 flux. Another problem with this design is that the wasted heat, due to larger voltage drop across the linear regulator 72 at low x-ray 417 flux, can heat surrounding electronic components, resulting in temperature fluctuations and instability in these electronic components.
High Voltage Multiplier Distributed Capacitance Power Loss
As shown in FIG. 8, a high direct current (DC) voltage generator 80, comprising an alternating current (AC) source 51 and high voltage multiplier 54 can have a power loss, shown as imaginary distributed capacitor 81. This capacitance, between an AC connection 54b and ground connection 54a can be large and can result in power loss as alternating current flows to and from the ground 53. It could be beneficial if the alternating current did not flow to and from the ground 53, or if alternating current to and from the ground 53 was substantially reduced, thus avoiding or reducing the large capacitive power loss between the high voltage multiplier 54 and ground 53. This power loss is wasted energy and can result in reduced battery life, for battery powered power supplies.